The present invention relates to test and measurement instrumentation, and more particularly to triggering of instruments for acquiring and analyzing data in real time.
FIG. 1 shows a basic block diagram of one type of a real-time test and measurement instrument—a real-time spectrum analyzer (RTSA), such as the Tektronix WCA280A, WCA380, RSA2200A and RSA3300A manufactured by Tektronix, Inc. of Beaverton, Oreg. An RF signal is input to an RF signal conditioner and then converted to an intermediate frequency (IF) signal by mixing with a local oscillator frequency. The IF signal is filtered and digitized, and the resulting IF data amplitude and phase are corrected. From the corrected IF signal a set of I (In-phase) and Q (Quadrature-phase) data may be generated for storage in a memory for subsequent processing. The I and Q data also may be input to a trigger generator. In response to specified conditions that define a trigger event, the trigger generator outputs a trigger signal. A memory controller in response to the trigger signal captures the I/Q data in the memory that surrounds the defined trigger event in the RF signal. The defined trigger event may occur in the frequency spectrum being monitored by the spectrum analyzer. The implementation of such a “frequency mask” trigger is disclosed in U.S. Pat. No. 5,103,402. Also conventional time domain triggers may be used that trigger based on the power (or voltage level) of the RF signal being monitored, as in conventional oscilloscopes.
Modern communication systems have become quite complex and include many different modulation systems for placing information on a carrier signal. As a result often a problem is known to exist, but the source of the problem is difficult to pin down. This is especially true when the problem is intermittent. For example in a demodulated bit sequence there may be an occasional wrong bit in the sequence. In order to solve such a problem it is necessary to capture data surrounding the anomalous event. Since the error is in the information or demodulation domain, frequency mask triggering and amplitude triggering are not suitable to find such an information error. Thus long data records, on the order of hours or even days, may have to be examined manually to try to find the data that surrounds such an event. The information may be presented in the form of a constellation display or error vector magnitude display for QAM modulated signals, and then identified from such a display after processing all of the long data records. In other words the communication system knows that there is a problem, but not where or when the problem occurs. Once the problem is located in time and space, then the problem can be solved from an analysis of the data occurring in the region of the problem event.
What is desired is to extend real-time triggering beyond frequency mask triggering and conventional time domain (amplitude) triggering to a method of automatically acquiring data immediately surrounding an anomalous or identifiable event in the information domain of a communication signal.